Detection circuit utilizing opposite conductiviity transistors to detect charge on acapacitor



.June 22, 1965 J. S DETECTION CIRCUIT UTILIZIN TONE G OPPOSITE CONDUCTIVITY TRANSISTORS T0 DETECT CHARGE ON A CAPACITOR 2 Sheets-Sheet 1 Filed Oct. 19, 1961 Flal BY lm/g J. STONE 3,191,058

DETECTION CIRCUIT UTILIZING OPPOSITE CONDUCTIVITY TRANSISTORS TO DETECT CHARGE ON A CAPAGITOR Filed Oct. 19, 1961 2 Sheets-Sheet 2 il J 1N VEN TOR. JACK f. mmf

p or rectangularrshaped signals.

UnitedStates Patent O 3,191,058 DETECTION CERCUIT UTLIZNG GPPGSITE `CUNDIUCTIVITY TRANSISTORS T() DETECT CHARGE N A 'CAPAClTOR Jack Stone, Ambler, Pa., assigner to Sperry Rand Corporation, New York, NX., a corporation of Delaware Filed ct. 19, 1961, Ser. No. 146,094 4 Ciaims (Cl. 307-835) This invention relates to a detection circuit, and more particularly to a circuit for detecting differences in pulse widths between two consecutive electrical signals in a signal train.

ln many electrical systems, such as a computer system, electrical signals having square or rectangular shaped characteristics are often involved. It is often desirable in such systems to detect variations in the Widths of two such consecutive signals.

An example of a use of such a circuit may be found in a phase modulation computer system. In such systems, binary information signals are recorded on a recording medium, such as a magnetic drum or tape. Such binary signals, having one of two different characteristics, may represent a 1 or a 0. A signal representing a "1, for example, may be represented by an alternating signal having a iirst form during the iirst half of its digit period and a second form during the second half of its digit period. Likewise, a 0 may be represented by a signal which is in the second form during the iirst half of its digit period and in the first form during the second half of its digit period. Both types of signals may be considered as passing through zero in going from one level to another at the middle of their digit periods.

After the recorded signals are read from the recording medium, they generally are passed through various electrical circuits and are converted to a form of square wave These square wave signals are then used to produce information pulse signals representing either a l or a 0. In generating inTormation pulse signals, spurious or non-significant pulse signals are produced whenever two consecutive information signals of a similar nature are read from the recording medium. Various additional means must generally be employed to eliminate these spurious signals before passing on the information signals to subsequent utilization circuits.

It is an object of this invention to provide a circuit for detecting non-symmetry between two consecutive signals in a signal train.

It is a further object of Vthis invention to provide an improved circuit for detecting the character of two consecutive binary signals.

It is a further object of this invention to provide an improved circuit for eliminating spurious pulse signals from a wave train of information pulses in a phase modulation reading circuit of a computer system.

In accordance with the present invention, a circuit is provided for detecting non-symmetry between two consecutive square wave signals. The square wave signals are applied to an integrator circuit which charges during the rst half cycle of each signal and discharges during the second half cycle. The rate of charge and discharge 3,191,058 Patented June 22, 1965 ICC of the integrator circuit is maintained relatively constant. If the nature of two consecutive signals are the same, the integrator circuit will not be charged at the end of the digit period or cycle. If the two consecutive square wave signals are different, the integrator circuit retains an electricalcharge. Means for detecting the charge at the integrator circuit is provided. Various utilization circuits may be actuated in accordance with the state of charge at the integrator circuit.'

Other objects and advantages of the present invention will be apparent and suggest themselves to those skilled in the art, from a reading of the following specification and claims in which:

FIGURE 1 is a schematic diagram of a detection circuit, in accordance with the present invention;

FIGURE 2 is a block diagram illustrating a reading circuit in a phase modulation system, in which the present invention may be employed, and

FIGURE 3 is a series of waveforms shown to illustrate the operation of the reading circuit of a phase modulation system such as illustrated of FIGURE 2.

Referring particularly to FIGURE 1, a rectangular signal illustrated by a waveform 10 is applied to an input terminal 12 of a detection circuit 14. A first pair of transistors. 16 and 18 and a second pair of transistors 20 and 22 provide means for coupling the input signal 10 to an integrator illustrated as being a capacitor 24. The transistors 16l and 22 are illustrated as being of the NPN types, while the transistors 18 and 20 are of the PNP types. As will be described, the capacitor 24 will assume a negative potential charge or a positive potential charge whenever the positive portion ofthe input signal 10 is not symmetrical with the subsequent negative portion of the cycle, i.e. they are not of the same duration or Width. For purposes of explanation, the upper portion of the waveform 10 will be considered positive with respect to a point of reference potential zero. Likewise, the lower portion of the waveform 10 will be considered negative. If the input signal 10 includes two consecutive symmetrical portions of equal width, the capacitor 24 willassume a zero charge.

A pair of transistors 26 and 28 and a pair of transistors 30 and 32 are provided for detecting the state of charge of the capacitor 24. The transistors 26 and 32 are of the PNP types While the transistors 28 and 30 are of the NPN types. Dependent upon the state of charge of the capacitor 24, i.e. whether it assumes a negative or positive charge, an output signal will be developed the collector electrode of either the transistor 26 or 30. A signal developed at the collector of either transistor 26 or 30 may be applied to actuate a utilization circuit, such as a bistable or Hip-flop circiut 34.

Assume first that the input signal 10 is in the negative state, i.e. at its lower level. At this point, the transistors 16, 18, 20 and 22 are biased to produce certain operating conditions. The transistor 16 is in an off or non-conducting state. The transistor 18 is in an on or conducting state. During the time, that the, transistor 18 is conducting, a relatively constant current charges the capacitor 24. When the input signal 10 is negative, the transistor 20 is on or in a saturated state and the transistor 22 is off or in a non-conducting state.

When lthe input signal 10 goes positive from a negative state, the transistors `16, 18, 2t) and 22 are switched to their opposite oper-ating states. The transistors 16r and 22 are switched from their oli states to on states and the transistor 18 and 2t) are switched from their on state to off states. 4Under the-se operating conditions, a Constant current linearly discharges the capacitor 24. if the signal cycle of the signal is symmetrical, i.e. the input signal 10 includes -two consecutive signal periods of opposite polarities and of the same duration or Widths, the voltage at the capacitor 24 at the end of the two consecutive periods will be Zero. If the signal l@ does not include two consecutive period-s of the same duration, i.-e. they are not symmetrical, the capacitor 24 will assume a positive or negative charge, depending on which of the two periods is longer in duration, y

When the time intervals of two consecutive signalrperiods are the same,l the capacitor'24 will charge during the first period and discharge during the second period. Since the time intervals of both periods are the same, the

. total charge on the capacit-or Z4 willbe the same at the end of the two periods as it was at the start'of the two periods. This charge will` generally be Zero voltage.'

When the two consecutivetime periods of the signal Vare of dilerent durations, a char-ge will'be developed at Athe capacitor 24. The capacit-or 24 will be discharged during the sampling intervals when signals 36 and 38'arc Vapplied to the bases of the transistors 28 and 32, respectively. The discharge will take place through ltransistors 26 or 30 `depending on Awhether C has a plus or a minus charge. Pulses are developed at the collectors of the transistors 26 or 39 during the sampling intervals and are applied ot the nip-hop circuit 34 when the capacitor 24 i-s ina state of charge.

Consider now the sampling circuit which may be used with the present invention. A pair of transistors 23 and 32 are normally olf or in their non-conducting states.:

The transistor 28 may be of the NPN type while the transistor32 may be of the PNP type. When the sampling'pulse signals 36 and 38 are Iapplied to the input terminals 40and 42, respectively, the transistors 28 and 32 are switched on to their saturated conducting state. Y

' the capacitor 24 discharges through the transistor 26 to produce a pulse signal at the collector electrode of the transistor 26. The pulse signal at thel collector of the metrical signal. While it is realized that the circuit disfare applied to the sampling circuits 64 and 66 in order to detect the state of charge of the capacitor 56. If the closed may have Wide application in systems where symmetrical and non-symmetrical signals must be detected to perform certain functions, the invention has found particular application in .a phase modulation reading circuit of a computer systems. Such a system will be generally considered to particularly point out one instance in which the present invention has proven to be useful.

Referring particularly to FIGURE 2, a reading arrangement for a phase modulation reading system is illustrated. A coil `46 may be associated with a reading hea-d of a tape or drum storage device, for example. During a reading operation, electrical `signals are produced in the coil 46 and applied to a pair of amplitiers 48 and Si). These signals may roughly correspond Vto `the signals represented by the Waveform A of FIGURE 3. The output signals from Athe amplifiers 48 and 50 are applied to a pair of Schmitt trigger circuits 52 and 54, respectively. The output from the Schmitt trigger circuits .are illustrated'by a waveform B.

The output signals from the Schmitt trigger circuits 54 and 52 are applied to capacitors 56 and 58, respectively. The output signalsY from the Schmitt trigger circuits 52 .and 54 are also lapplied to a pair of pulse generator circuits and 62, respectively, to produce a series of pulse signals. The output pulse signals from the pulse generator 60 is rep-resented by thewaveorm C. The output pulse signals from pulse generator circuit 62 are represented bythe waveform D.

The output electrical signals from the Schmitt trigger circuits 52 and 54 cause a charging and discharging of the capacitors 56 and 58, respectively, in a manner similar to that described in connection with FIGURE 1. The output pulse signals from the pulse generator 60 are applied to sampling circuits 64 and 66. Likewise, the output pulse signals from the pulse generator 52 are applied to the sampling circuits 78 and 80.

With the signals represented by the wavetrain B be- `ing applied from the Schmitt trigger circuit 52, the capacitor 56 charges and discharges at a rate illustrated by the waveform E. Likewise, the output signal from the Schmitt trigger circuit'54 causes the capacitor 58 to charge and discharge at a rate illustrated by the waveform F.

The output pulse signals from a pulse generator 60,

' state of charge of capacitor 56 is zero, no output signal ytransistor 26 may be appliedrto set the flip-flop 34.'. The

flip-op 34, in turn, may control the operation of an AND gate circuit 44. In the previous discussion, the transistor 26 may 4be considered as a form of AND gate circuit,

which produces an output signal when t-wo input signals are present. One input signal alone is not suicient to produce .an output signal.

If the voltage charge at the capacitor 24 is negative in polarity at the time of the sampling pulses 36 and 38, a pulse signal is produced at the collector of the transistor 30 and the capacitor 24V discharges through the transistor 30. This pulse signal may also. beV applied to the ip-op 34 to switch `or reset the flip-Hop. The gate circuit 44, being connected to the -set output of the flip-nop 44, will be opened or closed depending upon the operating state of the flip-op 34, which in turn is dependent upon the last state of charge of the kcapacitor 24.

The particular circuit disclosed embodying the present invention may be used in various applications. For eX- .ample, the circuit disclosed may be used in connectionY with servo systems. In this application the state of charge of the capacitor 24 may be employed to Vary the duration of the cycle of the input signal f1@ to provide a symwill be developed at line 67 or 83 to atiect the operating stateof a flip-flop circuit 68. On the other hand, if the state of charge of the capacitor 56v is other than zero, a pulse signal will be developed at the line 67 or S3 and applied Vt0 the liip-iiop circuit 68. For example, if a pulse signal is produced by the pulse generator 60' when the voltage charge atv the capacitor 56 is positive, a pulse signal,'illustrated by the waveform H will be applied to the flip-flop circuit 68. The output signal at the line 61 `of the llip-op circuit 68 is applied to a gate circuit V7). The output voltage from the flip-flop circuit 68 is represented by the waveform K. When the output voltage from the flip-iiop circuit 68 is at a lower or more negative level, the gate circuit 70 will be open to permit any pulses appearing from the pulse generator circuit 60 to pass through the gate 70 to an output terminal 72. When the output voltage from the flip-tiop circuit 68 rises to a higher level or more positive, the gate -circuit 70 will be closed thereby inhibiting the passage of any pulses from the pulse generator 60 to the output terminal 72. A short delay circuit 86 is necessary to inhibit the first non-significant pulse after a change of state of Hip-flop 68 and to permit passage of the first l information pulse.

Amplitude threshold circuits (not illustrated) may be necessary in some practical systems at the output circuits of the sampling circuits. These threshold circuits will prevent the flip-tlops 68 and 76 from being triggered when a relatively small plus or minus charge is accumulated by the integrator at the capacitors 56 or 58 as a result of undesired non-symmetry. Such non-symmetry may be referred to as jittery, Such threshold circuits are well known to those skilled in the art. Since they are not specifically directed to the invention, they are not illustrated for purposes of clarity.

The functions of the sampling circuits are two-fold. The rst function is to produce an output pulse when the integrator indicates that the input signal has changed phase by 180 degrees. The second function is to insure that the integration of cach cycle starts from the same reference level, i.e. independent of previous history.

In the system illustrated, it is desired to detect only phase changes of 180 degrees. However, the system illustrated could be used in other applications, to detect any phase change and produce a pulse in which the arnplitude of the pulse is proportional to the amount of phase change. The latter arrangement, of course, would not require the use of threshold circuits previously discussed.

Each time that the capacitor 56 is charged to some voltage level, it is discharged to zero volts during the sampling interval. This type of operation was described in connection with FIGURE 1. Pulses are developed to control flip-liep circuits 68 and 76 during the discharge of the capacitor 56 and 58 caused by the sampling pulses.

A second gating circuit 74 is controlled by the operating state of a flip-op 76. The operating state of the flip-flop 76, having an output waveform such as illustrated at point L, at the line 77. Output pulse signals from the pulse generator 62, illustrated by the waveform D, are applied to a pair of sampling circuits 78 and 80 to detect the state of charge at the capacitor 5S. If the state of charge at the capacitor 58, illustrated by the waveform F, is negative, a negative pulse, represented by the Waveform I, is applied from the sampling circuit 78 to the flip-hop 76. Under these conditions, the 0perating state of the flip-iiop 76 is switched to close the gate circuit 74. This prevents any pulses from the pulse generator 62 from passing through the gate circuit 74 t0 the output terminal 82. A delay circuit 88 is provided between the pulse generator circuit 62 and the gate circuit 74.

A positive charge on capacitor 58 is detected by sampling circuit 80 and a pulse is developed at a point I to reset or switch the operating state of the ip-liop 76 to thereby open the gate circuit 74. Opening of the gate circuit 74 permits pulse signals from the pulse generator 62 to be applied to the output terminal 82.

It is noted that the signal train illustrated by the waveform C includes pulses which represent true information or ls, as well as including pulses which do not represent the information. The latter type pulses may be classified as spurious or non-significant pulses. The signal train waveform M, representing the pulse signals passing through the gate 70 to the output terminal 72, on the other hand, only include pulses which represent true 1 bits of information and do not include nonsignicant 0r spurious pulses. The reason for this is that the gate 70 is open only during the time that the significant pulses are developed by the pulse generator 60 and is closed during the time interval involving the nonsignificant pulses.

Likewise, it is seen that the output pulses from the pulse generator 62, illustrated by the waveform D, include information pulses representing 0 information as well as non-significant pulses. The waveform P, however, includes only true 0 information pulses, the gate circuit 74 being open only during the proper time intervals in a manner described.

In relating the circuit of the present invention, illustrated by FIGURE 1, to the system illustrated in FIG- URE 2, the Schmitt trigger circuit 52 of FIGURE 2 may be considered as the circuits comprising transistors 16 and 26)'. Resistor 84 is the equivalent of the current source transistors 18 and 22. The sampling circuits 64 may be considered as including the transistors 26 and 28. The sampling circuit 66 may be considered as including transistors 30 and 32. Capacitor 56 to FIG- URE 2 may be considered the equivalent of the capacitor 24 of FIGURE 1. YThe flip-op circuit 34 of FIGURE 1 may be similar to the ip-op circuit 68 of FIGURE 2. The gate circuit 44 of FIGURE 1 may be similar to gate 76 of FIGURE 2.

It is noted that the circuit illustrated by FIGURE 1 would be used in connection with reading only one type of information, for example, 1 information signals. For a complete reading system involving reading both 0s and 1s, the circuit of FIGURE 1 would be duplicated to produce the system illustrated by FIGURE 2.

What is claimed is:

1. In combination with a source of input signals having first and second portions of a positive or negative level `with respect to a point lof reference potential, said positive and negative levels being of substantially the same voltage amplitudes with respect to said point of reference potential, means for detecting a difference in the ytime duration of said first and second portions of said signals comprising a capacitive circuit capable of assuming a positive or negative charge with respect to said po-int of reference potential, means for generating pulse signals each time said signals change from one level to the other, a transistor circuit for applying said input signals .to said capaotive circuit to charge said capacitive circuit during `said iirst portion of said input signal and to discharge said capacitive circuit during said second portion of said input signal, the rate of said charge and said discharge being substantially the same, said capacitive circuit retaining a positive or negative charge when the time durations of said rst and second portions of said input signal are different, said capacitive circuit retaining a zero charge when the time durations of said rst land second portions of said input signals are substantially the same, a detector circuit and means for applying said pulse signals to said detect-or circuit to detect the state of charge of said capacitive circuit.

2. In combination with a source of input signals having lirst and second portions of a positive or negative voltage level with respect to a point of reference potential, said positive and negative levels being of substantially the `sarne voltage amplitudes with respect to said point of `reference potential, means for detecting a difference in the time duration of said first and second por tions of said signals comprising a capacitive circuit capable of assuming a positive or negative charge with respect to said point of reference potential, means for generating pulse signals each time said input signals change from one level to the other, `a circuit for applying said input signals to said capacitive circuit to charge said capacitive circuit during said iirst portion of said input signal and to discharge said capacitive circuit during said second portion of said input signal, the rate of lsaid charge and said discharge being substantially the same, said capacitive circuit retaining a positive or negative charge when the time durations of said iirst and second portions of said input signal are dilerent, said capacitive circuit retaining a zero charge when the time durations of said iirst and second portions of said input Asignals are substantially the same, a transistor circuit normally biased to cutoff, means for applying said pulse signals to :said transistor circuit, means for applying a voltage at said -capacitive circuit to said transistor circuit, said transistor circuit becoming conductive when a voltage signal from said capacitive circuit and a pulse signal `are simultaneously applied thereto.

l3. The invention as set forth in claim 2 wherein said transistor circuit includes a pair of transistors, with one 7 8 transistor responsive to become conductive-for a positive References ited by the Examiner voltage charge at said capacitive circuit .and the other UNITED' STATES PATENTS transistor being responsive to become conductive for a I negative voltage chargerat said capacitive circuit. 3064144 .11/ 62 Hardy 307-883 4. The invention as set forth in claim 3 wherein said 5 31140'406 7/64' Thompson 307-885 pair of transistors each comprise emitter, collector and v OTHER REFERENCES base electrodes with Sad Pulse Sign@ 'being aPPHed to Pub 1: IBM Disclosure Bulletin lvol 3 No l0 March the 'base electrode, the voltage charge from said capaci- 1961 gage 83. Y v

tive circuit being applied to said emitter elect-rode, an output circuit, and means for applying an output sign-a1 10 ARTHUR GAUSS, Plm'C-fy Exfflmeh from said collector electr-ode Ato s-aid output circuit. JOHN W 'HUCKERT, Examl-M 

1. IN COMBINATION WITH A SOURCE OF INPUT SIGNALS HAVING FIRST AND SECOND PORTIONS OF A POSITIVE OR NEGATIVE LEVEL WITH RESPECT TO A POINT OF REFERENCE POTENTIAL, SAID POSITIVE AND NEGATIVE LEVELS BEING OF SUBSTANTIALLY THE SAME VOLTAGE AMPLITUDES WITH RESPECT TO SAID POINT OF REFERENCE POTENTIAL, MEANS FOR DETECTING A DIFFERENCE IN THE TIME DURATION OF SAID FIRST AND SECOND PORTIONS OF SAID SIGNALS COMPRISING A CAPACITIVE CIRCUIT CAPABLE OF ASSUMING A POSITIVE OR NEGATIVE CHARGE WITH RESPECT TO SAID POINT OF REFERENCE POTENTIAL, MEANS FOR GENERATING PULSE SIGNALS EACH TIME SAID SIGNALS CHANGE FROM ONE LEVEL TO THE OTHER, A TRANSISTOR CIRCUIT FOR APPLYING SAID INPUT SIGNALSTO SAID CAPACITIVE CIRCUIT TO CHARGE SAID CAPACITIVE CIRCUIT DURING SAID FIRST PORTION OF SAID INPUT SIGNAL AND TO DISCHARGE SAID CAPACITIVE CIRCUIT DURING SAID SECOND PORTION OF SAID INPUT SIGNAL, THE RATE OF SAID CHARGE AND SAID DISCHARGE BEING SUBSTANTIALLY THE SAME, SAID CAPACITIVE CIRCUIT RETAINING A POSITIVE OR NEGATIVE CHARGE WHEN THE TIME DURATIONS OF SAID FIRST AND SECOND PORTIONS OF SAID INPUT SIGNAL ARE DIFFERENT, SAID CAPACITIVE CIRCUIT RETAINING A ZERO CHARGE WHEN THE TIME DURATIONS OF SAID FIRST AND SECOND PORTIONS OF SAID INPUT SIGNALS ARE SUBSTANTIALLY THE SAME, A DETECTOR CIRCUIT AND MEANS FOR APPLYING SAID PULSE SIGNALS TO SAID DETECTOR CIRCUIT TO DETECT THE STATE OF CHARGE OF SAID CAPACITIVE CIRCUIT. 